Various devices for protecting electrical circuits, and equipment incorporated within such circuits, are of course well-known. For example, in U.S. Pat. 4,910,489 issued to Helmuth Neuwirth et al. on Mar. 20, 1990, there is disclosed a fail-safe secondary fuse device 20 for assuring the grounding of a conductive gas tube incorporated within modular protection devices for individual subscriber circuit pairs. The device comprises a length of spring music wire 21 which has a central loop adapted to surround the central electrode contact 19 of the gas tube 10, and rectilinear legs 23 and 24 which terminate in ends 25 and 26. The contacts 17 and 18 of the end electrodes 15 and 14 of the gas tube 10 are insulated from direct electrical communication with the respective ends 25 and 26 of the device 20 by means of a fusible sleeve 27 fabricated of suitable insulative material and incorporated upon each one of the ends 25 and 26 of the device 20. Upon the occurrence of a continued current overload condition or state, the heat emanating from the gas tube 10 will serve to fuse or melt the sleeves 27 whereby the ends 25 and 26 of the legs 23 and 24 will be permitted to short circuit the end electrodes 15 and 14 to the central electrode 16 which is grounded.
In U.S. Pat. 4,717,902, which issued to Kenneth S. James on Jan. 5, 1988, there is disclosed an excess voltage arrestor which is provided with a protective temperature responsive device formed from a wire 1 of spring temper. The wire 1 has a central loop portion 2 and is coated with a polyurethane varnish 3. In a manner similar to that of Neuwirth et al., the loop portion 2 is disposed about the terminal pin 4 of the central or intermediate electrode of the voltage arrestor, while the spring arms of the wire 1 are disposed in a stressed condition as a result of being gaged with the terminal pins 5 of the outer electrodes of the volt age arrestor. When the voltage arrestor 1 experiences or senses an elevated temperature due to overload conditions, the coating comprising the polyurethane varnish decomposes so as to permit the now bare wire 1 to establish electrical contact between the terminal pins 4 and 5.
In U.S. Pat. No. 4,858,059 which issued to Masahiko Okura on Aug. 15, 1989, there is disclosed a short-circuit device for use in connection with a gas-filled, triple-pole discharge-tube type arrester. The arrester is used in conjunction with telephone lines and includes a coil-like resilient short-circuit lead 7 which is spot-welded at its central portion to a lead 5 of an earth or ground electrode 4, and wherein the end portions of the lead 7 are disposed in forced engagement with leads 3,3 of line electrodes 2,2 as a result of the short-circuit lead 7 being mounted upon the electrode leads 3,3,5 in an entwined manner. The end portions of the lead 7 which are disposed in contact with the line electrode leads 3,3 are coated with a low temperature-meltable insulator 8,8.
Continuing further, in United Kingdom Patent Application Number 2,167,915 which was published on Jun. 4, 1986, a circuit protection arrangement is disclosed which includes a normally-closed switch contact 4 in the form of a resilient wire. The wire is tensioned so as to be biased toward the central contact earth pin 2 of double gas discharge tube 1 but is normally constrained into contact a contact pin 3 of an input line L1 or L2 by means of a fusible joint at, for example, point P1. When the temperature of the gas discharge tube 1 rises sufficiently so as to melt the fusible alloy or solder comprising joint P1 when, for example, a surge or transient signal occurs, the switch contact 4 is released from its constrained position so as to short the output terminals E1 or E2 to ground.
As is generally well-known, the purpose of overvoltage protection circuits, devices, or arrangements is to protect the expensive equipment with which the overvoltage protection circuits, devices, or arrangements are operatively associated. Such protection circuits, devices, or arrangements are commonly associated with, for example, telecommunication equipment which is operatively connected to the output side of a terminal circuit so as to protect such equipment against high voltage surges caused, for example, by lightning strikes on the subscriber line. With the advent of electronic circuits for use within telecommunication equipment, there has arisen a need for providing new types of overvoltage protection means for such electronic circuits since they cannot tolerate overvoltage levels which were permissible or tolerable heretofore.
As has been previously briefly discussed in connection with the aforenoted patents to Neuwirth et al., James, Okura, Phillips et al., a known conventional type of overprotection device in widespread use comprises the so-called three-element gas tube having a pair of laterally spaced apart end electrodes and a central electrode wherein the end electrodes are typically connected to a pair of output lines coupled to the telephone equipment which is desired to be protected against excessive voltage levels, while the central electrode is connected to earth ground. Upon the occurrence of a voltage force between the end electrodes or between either one of the end electrodes and the central electrode wherein the voltage force has a value which is greater than a predetermined potential level, the gas tube becomes electrically conductive so as to shunt the overvoltage to ground and thereby protect the telecommunication equipment from the potentially damaging excessive voltage levels. However, in the event of a sustained overvoltage, overcurrent condition or state, that is, where the circuits exhibit power crossing characteristics, the gas tube remains conductive and becomes overheated thereby causing a fire hazard. Accordingly, it is important that the gas tube shuts down safely so as not to leave the telecommunication equipment exposed to the damaging overvoltage, overcurrent conditions.
The prior art has therefore also developed various types of fail-safe devices, arrangements, systems, circuits, and the like for use in conjunction with gas tubes and other kinds of overvoltage protection means, such as, for example, air-gap arrestors and the like. One form of fail-safe arrangement comprises a temperature responsive device comprising a resilient, electrically conductive member which is normally maintained in a stressed condition by means of a heat-softenable material. However, when the heat-softenable material is used to normally hold the stressed resilient, electrically conductive member out of engagement with a cooperative tact and is melted so as to permit the establishment of a circuit arrangement between the conductive member and the cooperative contact, there exists the possibility that the stress applied to the conductive member will not be sufficient enough to cause the conductive member to properly engage the cooperative contact and thereby shunt the overvoltage potential to ground within a predetermined time interval.
In accordance with later technological developments of the prior art, the known or conventional three-element gas tubes have been generally replaced by means of-solid-state voltage suppressors commonly referred to as sidactors which have similar structural dimensions with respect to those characteristic of the gas tubes. sidactors are provided with a plurality of legs for enabling mounting of the sidactors within corresponding holes provided within a printed circuit board, and as a result, the telephone connector blocks incorporating the printed circuit boards therein can be fabricated with an even higher circuit density. It has therefore been necessary to provide an improved fail-safe shunt protection arrangement for assuring that sufficient pressure is applied to the shunt device or arrangement so as to in fact ground the sidactor at elevated temperature levels without significantly increasing the amount of space required.
The invention disclosed within the aforenoted parent patent application was directed toward the aforenoted desired improvement in order to in fact provide an improved fail-safe shunt protection arrangement or device for use in connection with sidactors. In particular, the improved fail-safe shunt protection arrangement was directed toward providing the requisite or sufficient pressure upon the contact pins of the sidactor when grounding of the sidactor is required under overvoltage, overcurrent elevated temperature conditions. More particularly, the invention of the parent patent application comprised a torsional type spring element whose spring arms were initially formed so as to have an obtuse angle with respect to each other, and when the spring element was mounted upon the sidactor, the spring arms were forced to assume a rectilinearly aligned position with respect to each other so as to place the spring arms under the requisite amount of tension. In addition, the center of the single loop portion of the torsional spring was disposed in an off-center relationship with respect to the centers of the sidactor terminal pins in order to further insure that sufficient and constant pressure was defined between the spring element spring arms and the end terminal pins. The ends of the spring arms are provided with insulators, and upon the occurrence of a sustained overload condition, the heat emanating from the sidactor will melt the insulators so as to short-circuit the end terminal pins with respect to the central terminal pin.
While the aforenoted sidactor and fail-safe mechanism of the parent patent application have proven to be quite satisfactorily operative under most overload conditions whereby, for example, the expensive telecommunication equipment, with which the sidactors and fail-safe mechanisms have been operatively associated, have been properly or adequately protected, it has been experienced that under certain overload conditions, such as, for example, at a voltage level of 600 VAC and a current level of 120 or 360 amps, the sidactor body member or packaging does not have sufficient strength to withstand such overload conditions or voltage and current levels and accordingly, its integrity is not always able to be maintained In particular, cracking of the sidactor body member or packaging has been experienced, and as a result of such cracking, plasma gas or a plasma cloud is able to escape or be released from the sidactor body or packaging. Such a plasma cloud is undesirable from an operational viewpoint for the sidactor because such plasma gas provides an environment in which undesirable arcing between the leads of the sidactor can readily occur.